In many applications, and especially for wheel shafts, gearbox output shafts or crankshafts of internal combustion engines, the rotating shafts are equipped with an angular displacement encoding device in order to determine their instantaneous rotation speed and/or their angular position and/or their direction of rotation.
The encoding function may be performed by one or more polarized magnetic marks that form an encoding zone, the displacement of which is detected by a magnetic field detector placed facing this encoding zone. For such an encoder, it is known, for example from FR-A 2 777 060, to produce the mark(s) in the encoding zone using a layer of “elastoferrite”, that is to say a polymer of the elastomer type containing a high proportion of magnetic particles such as, for example, ferrite powder particles, which is adhesively bonded to the sleeve of the encoder while it is being moulded. Such a solution is practical to implement owing to the formation of the magnetic layer and the polarized marks, and also the adhesion of this layer to the sleeve during the moulding process.
However, the intensity of the magnetic field produced by the elastoferrite is insufficient for some applications. To increase the intensity of the magnetic field created by an elastoferrite, it is known to increase the proportion of magnetic particles. However the density of magnetic particles achieved hitherto make the elastoferrite too brittle and friable for certain applications.
It is also known, for example from EP-A-1 517 149, to produce the marks in the encoding zone using a permanent magnet, that is to say for example made of a ferrite, or made of neodymium iron boron (NdFeB). Such a solution makes it possible to increase the intensity of the magnetic field produced. In that document, the permanent magnet is fastened to the sleeve by continuous or discontinuous peripheral crimping. This crimping may be carried out by locally deforming part of the cylindrical sleeve, at a number of spaced-apart points, against the permanent magnet. This crimping may also be carried out by deforming a series of tongues on the periphery of the sleeve against the permanent magnet.
However, all these permanent magnets are made of materials known for their corrosion sensitivity, but also for their brittleness, that is to say their sensitivity to cracking when exposed to excessively high mechanical stresses. These stresses may arise from crimping the sleeve against the permanent magnet, or from the vibrations of the shaft, or else from impacts from materials thrown up when a vehicle is being driven.